1. Chapter 2 – Data Link Layer Issues
Computer Networks
Chapter 2 – Data Link Layer Issues
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

2. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
LAN Properties
Shared medium
High data rate
Low delay
Low error rate
Native broadcast support
Limited physical extent (a few kms)
Limited number of stations (100’s)
STAs are peers
Local management (not under PTT regulation)
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

3. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
Medium Access
Access allocation so that
One STA at a time accesses medium
Each STA gets a fair share
Delays are reasonable
Overhead and waste are minimized
Approaches
Tokens
Contention
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

4. Token-based Approaches
Token Ring
STAs linked (simplex) to two neighbors
Token circulates physical ring
Add STA by insertion into ring
Token Bus
STAs attached to bus
Token circulates logical ring
Add STA to bus and insert into logical ring
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

5. Contention Approaches
ALOHA
Can’t sense medium, so just talk!
CSMA
Listen before talk
CSMA/CD
Add collision detection (need sensitive PCS)
CSMA/CA
Use collision avoidance (when VCS used)
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

6. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
IEEE 802
802.1 – common issues
Addressing, management, bridging, security
802.2 – LLC
Type 1 – best effort / Type 2 - reliable
802.3 – CSMA/CD LAN
From Xerox Ethernet
802.4 – Token Bus
802.5 – Token Ring
– Wireless LAN
– Wimax
Note – FDDI standardized by ANSI
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

7. Names, Addresses, Routes
Name – what
Location-independent identifier
May be human-friendly or not
Address – where
Independent of source location, but will change if destination moves
Route – how to get there
Depends on both source and destination
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

8. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
LAN Addresses
LANs are broadcast medium – need both
Source address (for return messages)
Destination address (to filter)
IEEE 802 addresses
16 and 48 bits (also 60 for DQDB)
48 bit addresses managed by IEEE
Pay to get 224 address block, Vendor Code (OUI)
G/L bit is 0 if global, 1 if locally managed
OUI
2nd Octet
3rd Octet
4th Octet
5th Octet
6th Octet
G/L bit
(global/local)
G/I bit (group/individual)
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

9. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
Multicast Addresses
Service Discovery
Solicitation (client mcast to Z-Servers address)
Advertisement (Z-Servers mcast to Z-Client addr)
Why Multicast (group) addresses?
Reduce interrupt handling by hosts
Hardware filter
Why G/I bit?
Allow filtering by hash buckets in HW
SW filters all hits in relevant hash buckets
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

10. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
Broadcast
The all 1’s address (0xffffffffffff) is bcast
Means that all STAs must receive
Really, though, only those implementing the protocol used in the broadcast packet have to…
Broadcast address interrupts everyone anyway
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

11. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
Multiplexing Field
Multiple higher layer protocols
Format and data alone cannot distinguish
Multiplexing field selects which one
Protocol type in Ethernet
DSAP and SSAP in IEEE 802
Service Access Point (SAP) Structure
G/L and G/I bits also, hence 6 bits
All 1’s = all SAPs (!!!)
Others assigned by IEEE – too few!
SNAP (Subnetwork Access Protocol)
When DSAP = SSAP = 0xaa
extra protocol type field (5 octets)
3 OUI octets, 3 vendor-assigned octets
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

12. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
Bit Order
Bit order is order in which bits are put onto the medium
Not standard ()
LSB first canonical and for most LANs
MSB first for 802.5, FDDI
Bridges must convert
Shuffle bit order within octets
Impact on ARP and higher layer protocols
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

13. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
LLC
802.2 LLC has two significant types
Type 1 – datagrams (best effort)
Type 2 – reliable (connection oriented)
Type 1 Control – 1 octet
UI – unnumbered info (datagram)
XID – Exchange ID (command/response)
ID of transmitter
LLC types supported
Test – (Cmd/Rsp) – Rsp echo data in Cmd
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

14. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
LLC Type 2
Control is 1 or 2 octets, per type
2-octet control fields contain 7-bit seq #s
I = Information (data) – SN plus ACK SN
RR = Receive Ready (ACK) – ACK SN
RNR = Rcv Not Ready (Busy) – ACK SN
REJ = Reject – ACK SN
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

15. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
LLC Type 2
Control is 1 or 2 octets, per type
1-octet control field types
SABME = Set Asynchronous Balanced Mode Extended (start connection)
DISC = Disconnect (end connection)
DM = Disconnected Mode (confirm DISC)
FRMR = Frame Reject (receipt of invalid pkt)
UA = Unnumbered ACK (for DISC/SABME)
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

16. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
802.3 Issues
CSMA/CD
Listen before talk
Detect collisions
Binary exponential backoff
Minimum frame length
So all STAs detect collision
Slot time = 2t (512 bits at 2.5km and 10Mbps)
Start Tx
End Tx
Start Tx
Start Rx
Detect
Collision
End Tx
End Rx
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

17. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
802.5 Issues
Each packet traverses every station in physical ring
Each STA has transceiver buffer with special HW to recognize token, addresses
Two bits at end of each frame for ACK:
A bit (address recognized)
C bit (frame copied)
Each STA may modify bits
Sender sees A/C bits when frame returns
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

18. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
802.5 Issues
A/C Bits on return:
A=0: Address not recognized (give up)
A=1, C=0: Address recognized, but STA busy (try again)
A=1, C=1: Address recognized and frame copied (success!)
What does a bridge do with these?
Clear both?
Leave unmodified
Set A and C if bridge forwards
Clear A and set C if bridge forwards
A/C used for other purposes:
Ring order (bcast frame with A bit clear indicates predecessor)
Only 31 functional addresses for multicast
Mapping
Oversubscription
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

19. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
Packet Bursts
Station processing rate may be slower than the LAN data rate
While OK on average, packets may be sent in a burst
Early packets received, later ones lost
Problem if naïve protocol retransmits whole burst every time
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

20. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
Why Bridges?
Limit number of stations in LAN
Packet lengths
Delay
Size limitations
802.3 collision detection
Traffic
Capacity is shared
Simple, high performance, allow limited location transparency (keep IP address)
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

21. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
Point-to-Point Links
flag
address
control
data
checksum
flag
HDLC format
Flag
Special pattern to delimit frames
In HDLC,
In DDCMP, DLE-SOF & DLE-EOF
Bit-stuffing/character-stuffing for data transparency
In HDLC, … -> … on Transmit
In DDCMP, … DLE … -> … DLE DLE … on Transmit
Addressing
Needed if multiple stations on medium
Traditionally assume master/slave
Control – Like LLC Type 2
Checksum – 16 bit CRC
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

22. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
Point-to-Point Links
flag
Addr=0xff
Ctl=0x03
protocol
data
checksum
flag
PPP format
Multiplexing
Protocol field in PPP (16 bits – see RFC 1700)
Service
If links not reliable, need reliable transfer per hop (HDLC, DDCMP, LLC Type 1)
What is probability of success for k hops with FER P?
What is cost per attempt for k hops with FER P?
What is overall cost for success for k hops with FER P?
If links reliable, then datagram service OK (PPP, LLC Type 2)
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

23. Link Reliability Issues
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

24. Link Reliability Issues
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

25. Link Reliability Issues
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

26. Link Reliability Issues
CEN 5501C - Computer Networks - Spring UF/CISE - Newman

27. Per Hop vs. End-to-End BEC
Pkt Sent
Pkt Sent
Pkt Recv
Pkt Recv
ACK Recv
ACK Recv
Per Hop
End-to-End
CEN 5501C - Computer Networks - Spring UF/CISE - Newman